Pipe couplings are commercially available from a variety of manufacturing sources and are used extensively for underground piping installations. It is known in such couplings to employ gripping members of sorts, in order to prevent in-service uncoupling of the joint by longitudinal drawbar forces imposed on the coupled pipe section.
With the advent of plastic pipe and tubing and their approval by regulating governmental agencies for natural gas transmission, the previous problem of pipe pullout from a coupled joint has increased many times over to on the order of ten fold as compared to metal pipe. One factor most contributing to this problem is the much greater linear coefficient of thermal expansion for plastic pipe as compared to steel pipe. By way of example, Aldyl "A" polyethylene piping marketed by DuPont, has a linear coefficient of thermal expansion stated by the manufacturer to be: 9.times.10.sup.-5 in./in.F..degree. as compared to 6.5.times.10.sup.-6 in./in.F..degree. for steel pipe. Notwithstanding, government agencies regulating installation of such systems require that each joint sustain the longitudinal pullout or thrust forces caused by contraction or expansion of the piping or by anticipated external or internal loading to within prescribed limits. (Title 49, Section 192 "Transportation of Natural and Other Gas by Pipeline--Minimum Safety Regulations"--Fed. Vol. 35 NO 161).
Consequently, where such pipe sections are of plastic composition, they must not only be retained by the coupling in opposition to any longitudinally imposed draw bar forces without literally being torn away, but they must be retained to the extent that failure from these imposed forces will of necessity occur elsewhere in the system removed from the joint. A stab-type coupling for retention of plastic tubing is disclosed for example in prior U.S. Pat. No. 4,229,025.
In addition to the foregoing considerations, each manufacturer of such couplings instinctively strives to reduce product cost by improvements however marginal that result in cost savings in the end-product. While cost saving is a fundamental objective underlying competitive markets, it has generally been unknown how to achieve cost reductions in such couplings while maintaining the reliability of performance required to conform with the regulation standards therefor.